Digital subscriber lines are twisted copper pair lines with Digital Subscriber Line (DSL) modems operating at both ends of the line. This permits data to be communicated over a line at much greater data rates than is achievable with old voice-band modems operating over the same twisted copper pairs and is generally therefore referred to as “DSL broadband”. The DSL modems operate in accordance with various DSL standards agreed by the International Telecommunication Union (ITU). Over time newer DSL standards have been developed which permit ever greater data transmission rates to be achieved over digital subscriber lines. Furthermore, there is a desire to avoid the need to fit a service specific faceplate, thereby enabling DSL broadband to be provided on a self-install basis, avoiding the need for an engineer to visit the customer premises, keeping costs down to a minimum. The presence of home wiring then results in bridged taps being present in the circuit. These bridged taps cause deep zeroes in the insertion loss characteristic (i.e. in the amount of attenuation as discussed below), making it difficult to ascertain the loop length accurately for the purposes of upstream power back off Power Spectral Density (PSD) control (upstream power back off is discussed in greater detail below).
The physical length of twisted copper pair lines between a network side DSL modem located in, for example, a local Exchange or Central Office, or indeed in a street cabinet or a drop point (in VDSL systems it is often envisaged that the DSLAM may be placed much closer to the customer's premises than in the exchange) and a customer's premises may vary considerably from line to line. In general, the longer a line is, the greater will be the attenuation of a signal transmitted over the line by the time the signal reaches the far end. Furthermore, other factors in addition to the physical length of the line may affect the amount of attenuation suffered. This leads to the concept of the electrical length of a line, generally referred to in DSL literature (such as, for example, VDSL2 ITU standard G.993.2.) as kl0. In order to control the adverse impact of non-reciprocal crosstalk in the upstream direction, steps must be taken to equalise the received powers at the upstream VDSL CO or cabinet modem. In general the greater the electrical length, kl0, of the line, the more power with which a signal needs to be transmitted onto the line in order for it to be correctly detectable at the far end.
A particular problem experienced by DSL connections operating at increasingly higher frequencies (e.g. the VDSL standard uses higher frequencies than say ADSL1, etc.—in particular VDSL2 uses an additional band of frequencies for upstream transmissions at much higher frequencies than those used for upstream transmissions in ADSL for example) is known as FEXT (Far End CROSS Talk). This is noise (carried onto an adjacent line just before the lines reach a common receiver device) which is caused by signals transmitted at the far end of the common receiver device end (especially when the common receiver device is say a DSLAM and the lines are in a common binder connected to the DSLAM located in, for example, a Local Exchange or Central Office, or a cabinet or at a drop point, etc.). The noise is created by a customer premises DSL modem on an adjacent line—note that the interfering and the “victim” customer premises DSL modems might be somewhat distant from each other—in fact particular problems arise when the interfering modem is rather close to the DSLAM whereas the “victim” modem is located much further away from the DSLAM).
FEXT is a particular issue for VDSL2 because in VDSL2 (as specified in ITU standard G 993.2) a second upstream band of frequencies is used from approx. 3.0 MHz to 5.1 MHz, and, as mentioned above, FEXT becomes a more significant problem at higher frequencies. G 993.2 therefore specifies that DSL modems perform an estimation of the electrical length, klo, of the line over which the DSL connection is to be made, before such a connection is fully established. Furthermore, in many regions local PSD regulations, such as the UK Access Network Frequency Plan, specify that based on this estimation, an Upstream Power Back Off (UPBO) mask should be set to control the power used for upstream transmissions carried in the second upstream band of frequencies from approximately 3.0 MHz-5.1 MHz. In this way customer premises modems which are very close to the exchange should transmit at lower power levels compared to modems which are located much further from the exchange so as to minimise the FEXT impact of the “short” lines on “long” lines.
The exact way in which this parameter is determined is not currently specified by the standards but rather it can be determined in any manner deemed appropriate by DSL modem manufacturers. Indeed the standard seems to anticipate that both the network-side DSL modem and the customer side DSL modem may make an estimation of kl0 (since there is provision for both of these devices to send a message containing such an estimation from one device to the other) but no guidance is given as to which of these estimations should be used by the customer side DSL modem in order to generate its UPBO mask, or whether, for example, they should be combined in some way. The standard does include a note (see G 993.2 section 7.2.1.3.2) suggesting one possible approach for estimating kl0 (this method involves identifying the insertion loss at an unspecified number of different frequencies (provided they are within the range of frequencies between 1 MHz and the maximum usable VDSL2 frequency applicable to the modem—and selecting the minimum value of the ratio of the insertion loss to the square root of the frequency at however many different frequencies are selected for evaluation of this ratio, which could be just one). In general, however, many modems calculate this value based on only one or very few measurements at different frequencies of the amount of the insertion loss (i.e. the attenuation experienced by signals received over the line).